The current annual global energy demand of ˜14 terawatt-years (TW-yrs) is expected to double by mid-century and triple by the end of the century. Such a large increase in energy demand cannot be met by the existing carbon-based technologies without further destabilizing the climate. The sun is the largest source of carbon-free energy (120,000 TW-yrs strike the earth's surface annually) and can be used to produce both electricity and fuel. Yet in the United States, solar electricity (e.g. photovoltaics) and solar-derived fuels (e.g. biomass) currently provide about 1 millionth of the total electricity supply and less than 0.1% of total energy consumption, respectively.
An area of great promise for low-cost solar energy conversion is inorganic thin-film photovoltaics (PV). Thin-film PV has the potential to revolutionize the photovoltaics industry via cheaper processing and eliminating the use of expensive silicon wafers that account for over 50% of total manufacturing cost of traditional silicon-based PV. Current thin-film technology is based on amorphous silicon, CdTe, and CIGS (copper indium gallium diselenide) as the active absorber layers. These materials can be made 50-100 times thinner than traditional silicon cells because of their larger optical absorption coefficients. The resulting lower cost per peak watt ($/Wp) is driving the extraordinary market growth of thin-film PV, which is projected to account for 28% of the solar market by 2012 (at $19.7 billion in sales). CdTe and CIGS are currently the most favored of the thin-film technologies due to their high laboratory cell efficiencies (16.5% for CdTe and 19.9% for CIGS) and because amorphous silicon encounters certain stability problems. However, the future market share and societal impact of CdTe and CIGS PV will be limited by the scarcity of tellurium (Te) and indium (I) in the Earth's crust. Most projections conclude that price constraints on tellurium and indium will limit CdTe and CIGS to 0.3 TWs or less of total solar conversion capacity, which falls far short of the tens of terawatts of carbon-free energy that are needed to meet the global energy challenge. To enable the rapid expansion of PV to the multi-TW scale, it is essential to develop alternative thin-film PV materials based on common (rock-forming) elements and inexpensive manufacturing processes.
Iron pyrite (cubic FeS2) is experiencing renewed interest as an earth-abundant, nontoxic absorber layer for scalable thin-film photovoltaics (PV). Pyrite has an appropriate band gap (Eg=0.95 eV), very strong light absorption (α>105 cm for hv>1.3-1.4 eV), and sufficiently long carrier drift and diffusion lengths to produce large short-circuit photocurrent densities (>30 rnA cm-2) in photoelectrochemical and solid state Schottky solar cells based on pyrite single crystals. The main limitation with pyrite is the low open-circuit voltage (VOC) of pyrite devices, which does not normally exceed 200 mV, or ˜20% of the band gap. Efforts to correct this low VOC require basic studies of high-quality bulk and thin film pyrite samples.
Pyrite thin films have been fabricated by a variety of solution-phase and gas-phase methods. Solution methods that leverage atmospheric-pressure, high-throughput, large-area processing techniques like printing, roll coating, slit casting, or spraying may offer cost and scalability advantages relative to the vacuum-based batch processing traditionally employed in PV manufacturing. Solution methods used to make pyrite thin films include spray pyrolysis, chemical bath deposition (CBD), electrophoretic deposition (EPD), and sol gel chemistry. The strategy adopted in most of these cases is to deposit a film of (often amorphous) iron oxides or iron sulfides and anneal the fi m in sulfur gas at elevated temperatures (350-600° C.) to produce polycrystalline pyrite.
Table 1 compiles the principal reports of solution-deposited pyrite films, listing only those examples that provide substantive optical or electrical characterization of nominally phase-pure samples. Although many of these reports are partial and some present electrical data that is difficult to reconcile with results from other films and pyrite single crystals, most conclude that unintentionally-doped, solution-deposited pyrite films are p-type with low resistivity and low mobility, in agreement with results from samples grown by gas-phase methods. Recently, pyrite films have also been made by the solution deposition of pyrite nanocrystals, either with or without post-deposition sintering to increase grain size and film density, but the electrical properties of these films have not been reported in detail (see Table 1).
TABLE 1Synthesis and Properties of Solution-Deposited Pyrite Thin FilmsMethodPrecursersConditionsReported PropertiesRefspray pyroaq. FeCl3,550° C. in air (?),Eg = 1.05 eV6thioureano anneal (?)spray pyroaq. FeCl3,350° C. in N2 + S,Eg = 0.82 eV, p-type,a7thioureano anneallow mobility, p = 0.16 Ω cmspray pyroaq. FeSO4,120° C. in air,p-type,b p = 1016-1020 cm−3,9(NH4)2S500° C. anneal in Sμ = 1-200 cm2 V−1 s−1 (?)spray pyroaq. FeCl3350° C. in air,Eg = 0.73 eV, p = 0.6 Ω cm,10450° C. anneal in Snon-Arrhenius T dependencespray pyroaq. FeCl3350° C. in air,Eg = 0.93 eV, p-typea8350° C. anneal in Selectrodepaq. Na2S2O3,60° C.,n-type (due to Ti doping?)a13(NH4)2Fe(SO4)2500° C. anneal in Selectrodepaq. FeCl2,25° C.,Eg = 1.34 eV, p-type,b12Na2S2O3500° C. anneal in Sp = 1014 cm−3, μ~200 (?) CBDFe(CO)5, S80-165° C. in argon,photoactive17in org. solv.no annealCBDaq. FeSO4, en,28° C.,Eg = 0.94 eV, n-type (?)18Na2S2O3450° C. anneal in SEPDaq. FeCl3,200° C., no annealEg = 1.19-1.40 eV,19thiourean-type (?)sol gelaq. Fe(NO3)325° C., 500° C. annealEg = 0.99 eV, p-type,b20in air + 400° C. in Sp = 1019 cm−3, μ = 1.5sol gelFe(NO3)3, PO25° C., 100° C. annealEg = 0.93 eV22in EtOHin air + 450° C. in Ssol gelFe(NO3)3, 40° C.,Eg = 0.77-0.87 eV,21acac in EG500° C. anneal in airn-type w/low T anneal,400-600° C. in Sp-type w/high T annealbNCFeCl2, TOPO220° C. in argon,Eg = 0.93 eV, p-type,b25oleylaminedip coatingμ = 80 cm2 V−1 s−1 (?)molecularFe(acac)3 + S25° C., 320° C. annealEg = 0.87 eV, p-type,athis inkin pyridinein air, 390° C. in H2S +low mobility, p = 1.9 Ω cmwork550° C. in Sspray pyro = spray pyrolysis;electrodep = electrodeposition;CBD = chemical bath deposition;EPD = electrophoretic deposition;NC = nanocrystal deposition.DEG = diethylene glycol;en = ethylenediamine;PO = propylene oxide;EG = ethylene glycol;acac = acetylacetone;TOPO = trioctylphosphine oxide.(?) = incomplete or questionable data or conclusions.a = by thermopower;b = by Hall effect.
Therefore, high-quality bulk and thin film pyrite and processes to generate the same are desirable.